1. Field of the Invention
The present invention relates to a liquid crystal display device making use of a thin liquid transistor (TFT), such as a liquid crystal projector, and more particularly to improvements in light-shielding capability of an active matrix type liquid crystal display device for a light valve wherein switching of a liquid crystal is carried out through a TFT. Further, the present invention relates to a manufacturing method thereof.
2. Description of the Related Art
In recent years, as a display for the wall-hanging type television, the projective-type television or the OA appliances, various display units using a liquid crystal panel have been being developed. Among those liquid crystal panels, an active matrix type liquid crystal display wherein TFTs are included as the active element in a liquid crystal display device is the most promising to realize a high quality display unit for the OA appliances as well as a display unit for the high definition television because of its advantageous natures such as the one that an increase in number of scanning lines therein does not result in a deterioration of the contrast or the response time thereof. Especially when applied to a projection type liquid crystal display with the liquid crystal projection or the like, it allows achieving a large screen display with ease.
Normally, in the active matrix type liquid crystal display device for a light valve that is utilized for the liquid crystal projection, a small element is illuminated with a strong light, and the light passing therethrough is controlled according to the image data by turning on and off each pixel separately through switching of a liquid crystal by a corresponding TFT, and then the transmitted light, being magnified by an optical element such as a lens or the like, is projected on a screen or the like. At this, if an active layer of the TFT is formed from polysilicon (p-Si), there arises a problem of the optical leakage current in a channel section of the TFT at the off-time that may be produced, due to the optical excitation, by the reflected light from the optical system such as a lens, not to mention under the direct influence of the incident light thereon. Such an optical leakage current causes various display troubles including the appearance of streaks and flickers.
In the conventional active matrix type liquid crystal display device for a light valve of this sort, gate lines 7 and data lines 10 are arranged in the form of a matrix so as to intersect at right angles one another, as shown in FIG. 4, and in the areas demarcated with gate lines and data lines, transparent electrodes made of ITO (Indium Tin Oxide) 18 or the like, which serve as pixel electrodes, are formed, and in the regions where the gate lines 7 and the data lines 10 intersect, TFTS are provided. FIG. 5 is an enlarged view of the portion encircled in FIG. 4, showing a formation area of the TFT. On a data line 10, there is formed a contact 16 between the data line and the TFT to supply signals for a source region of the TFT. Through another contact 17 between the ITO and the TFT, a drain electrode 8 is connected with the ITO that is a pixel electrode. LDD (Lightly-Doped Drain) regions 15 are formed between a channel section (the part covered by the gate line) and source-drain regions. Further, FIGS. 6(a) and 6(b) are cross-sectional views taken on line IIIxe2x80x94III and line IVxe2x80x94IV of FIG. 5, respectively. In the structure shown in the drawings, there are provided an underneath light-shielding film 3 set over an underlying insulating film 2 on a transparent insulating substrate such as a glass substrate 1 or the like, and a black matrix 12 set above the TFT. Upon this, when the incident light comes from the side of an opposite substrate facing the TFT over a liquid crystal layer, the black matrix 12 cuts off the incident light and the underneath light-shielding film 3, the reflected light from the optical system.
The black matrix 12 is formed, in some cases, on the same substrate as the TFT with an interlayer film inserted therebetween as shown in FIG. 6, and, in other cases, on the opposite substrate to the TFT with the liquid crystal layer inserted therebetween. In the case that the black matrix 12 is formed on the opposite substrate to the TFT, the black matrix 12 must be made larger than the underneath light-shielding film 3 to allow 10 xcexcm or so for a shift in the overlay accuracy between two layers of the substrates. This brings about a problem that the aperture ratio cannot be made sufficiently large.
As a result, the method in which the black matrix as a second light-shielding film is formed on the identical substrate with the TFT is mainly employed at present. In this instance, it is unnecessary to allow such a large margin as described above since a high alignment accuracy can be obtained by making a good use of a manufacturing process of a semiconductor device. However, this method pays no regard to the positioning relation among these two light-shielding films and the TFT so that measures of cutting off the light caused by diffused reflection within the panel are not satisfactory. Nevertheless, in the gate-line formation region shown in FIG. 6(b), because the underneath light-shielding film 3 as well as the black matrix 12 is formed therein, the light is well cut off. However, in the region shown in FIG. 6(a) where no gate electrode is present, or in the region positioned between two pixel electrodes, the underneath light-shielding film 3 and the black matrix 12 are both restricted in width to increase the pixel aperture ratio. Accordingly, in the regions of a channel that is made of polysilicon and separating source-drain, wherein these electrodes and the gate line do not overlap, in other words, in the LDD regions 15, an adjustment of the widths of the black matrix 12 and the underneath light-shielding film 3 is normally made so that the incident light from the edge section of the black matrix may not reflect on the surface of the underneath light-shielding film 3. Yet, this measure cannot cut off the reflected light travelling from the optical system sufficiently. The reflected light passing over the edge section of the underneath light-shielding film 3 may enter into the LDD regions 15 through a multiple reflection among the black matrix 12, the data line 10 and the underneath light-shielding film 3, and cause the current leakage. Hereat, it should be noted that the directional components of the incident light or the reflected light are not only the component parallel to the direction of the gate line as shown in this example. They include components of various directions so that there are occasions of incidence even on the channel region beneath the gate line.
As measures against the reflected light from the optical system, it is proposed, in Japanese Patent Application No. 354845/1998, that an end section of an underneath light-shielding film is made into the form of a taper and the positioning relation between the underneath light-shielding film and an overlying light-shielding film that also serves as a data line is defined. Further, in Japanese Patent Application No. 109979/1999, there is disclosed a structure in which, in the vicinity of both lateral faces of the TFT in the direction of the channel length, dummy contact holes are formed on an interlayer film that is formed over an underneath light-shielding film, and films made of a wiring material are formed around the sidewalls of the dummy contact holes, and thereby the light such as the reflected light is prevented from entering into the channel or the LDD regions.
While both of these methods are effective in their own way, both require an additional step of a manufacturing method to form their characteristic structure, and obviously there remains some room for further improvements, especially from the point of production cost reduction.
Accordingly, an object of the present invention is to provide a liquid crystal display device such as a light valve, which is, with the pixel aperture ratio thereof set as large as possible, capable to prevent a reflected light travelling especially from an optical system from entering into a channel therein, without adding an extra step of a manufacturing method thereof.
In light of the above problems, the present inventor conducted investigations into the mechanism of the multiple reflection by the data line and the black matrix, and found out that the probability of incidence of the reflected light from the optical system becomes the smallest when, in the active-layer formation area of the TFT, the data line is formed to have substantially the same width as the underneath light-shielding film.
Accordingly, the present invention relates to a liquid crystal display device having, on a transparent insulating substrate:
an underneath light-shielding film;
a thin film transistor (TFT) in which, over an interlayer film on said underneath light-shielding film, an active layer made of polysilicon, a gate insulating film and a gate electrode connected with the gate line are successively formed;
a data line that feeds the data signals into said TFT; and
a black matrix formed over said data line to cut off the incident light;
with said active layer of the TFT being formed in the region where said gate line and data line intersect each other; wherein:
the underneath light-shielding film and the data line are formed to have substantially equal widths, at least, in said active-layer formation area.
Further, the present invention relates to a method of manufacturing a liquid crystal display device; which comprises the steps of forming, on a transparent insulating substrate, in succession, an underneath light-shielding film, a first interlayer film, a polysilicon that is to serve as an active layer of a thin film transistor (TFT), a gate insulating film, a gate line containing a gate electrode section, a second interlayer film, a data line, a third interlayer film and a black matrix; wherein:
the underneath light-shielding film and the data line are formed to have substantially equal widths, at least, in said active-layer formation area.
In the present invention, by forming the data line and the underneath light-shielding film to have substantially equal widths in the active-layer formation area, the probability of incidence of the reflected light from the optical system or the like becomes the lowest, and, therefore, the reduction of the optical leakage current can be achieved. Further, since the above structure can be obtained by changing only the mask pattern at the time of patterning of the data line, the steps of the manufacturing method may not become complicated, and, thus, an increase in cost for formation of the light-shielding structure can be well avoided.